Circular dichroism studies of the B→A conformational transition in seven small DNA restriction fragments containing the Escherichia coti lactose control region

Abstract

The B→A conformational transition caused by high ethanol concentrations was studied for seven DNA restriction fragments with overlapping and known sequences. Since the DNAs are homogeneous and range in GC content from 44 – 53%, they permit an evaluation of the influence of DNA sequence and base composition on the B→A transition. Moreover, their small size (80–301 bp) minimizes precipitation artifacts. The B- form spectra (in low salt) and the transition toward the C- form (in ethanol concentrations below the B→A transition) agree with prior measurements on chromosomal DNAs and are similar for all seven DNAs. At higher ethanol concentrations (80%), all fragments undergo a transition to the A- form as judged by the large increase of the positive CD band at 270 nm. Difference spectra among the fragments reveal minor differences between the A- form spectra. The ethanol concentration necessary to cause this transition is 72 ±2% for all fragments, thus excluding a preference of the CAP-, E. coli RNA polymerase-, or lac repressor-binding sequences for A- form. The kinetics of the B→A transition in 80% ethanol are biphasic; the initial rapid transition is an intramolecular B→A form shift and the slower transition is an aggregation (but not precipitation) of the DNA.